The present invention relates to an engine and in particular, although not exclusively, to a two-stroke reciprocating piston engine.
It is known to provide a crankcase-scavenged two-stroke engine comprising a piston which reciprocates in a cylinder, the cylinder having transfer ports from the crankcase to the cylinder, and an exhaust port. The top of the exhaust port is located higher up the cylinder than the transfer ports, so as to permit most of the combustion gases to escape before a new charge enters the cylinder via the transfer ports. In other words the exhaust port is uncovered by a descending piston before the transfer ports. A subsequent charge enters the crankcase on the upstroke of the piston, and is pushed into the cylinder when the transfer ports reopen on the next down stroke of the piston.
Several problems are associated with the prior art crankcase-scavenged two-stroke engines. The requirement for the transfer ports to be in the swept stroke represents an inefficiency of the induction cycle since little or no work can be obtained from the piston displacement when the transfer ports are open.
It is well known that fresh charge can pass directly to exhaust, and it has been proposed to provide a tuned exhaust in an attempt to push the escaped charge back into the cylinder by the use of pressure pulses but this can result in an engine with a narrow power band.
The exhaust and transfer port design of prior art two-stroke engines is typically a compromise which may reduce the theoretical maximum power output from the engine and may also contribute to increased emissions from the engine.
These problems are all well known, and numerous solutions have been proposed to improve engine efficiency, to reduce contamination of the charge due to crankcase lubricant and to reduce pollution due to unburned fuel leaving the exhaust port.
What is required is an improved engine which can overcome the aforementioned problems, and maximise the opportunity for charge pumping and charge compression, and reduce transfer losses in a simple and cost effective manner.